/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.server.handler;

import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.web.server.ServerWebExchange;
import org.springframework.web.server.WebFilter;
import org.springframework.web.server.WebFilterChain;
import org.springframework.web.server.WebHandler;
import reactor.core.publisher.Mono;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.ListIterator;

/**
 * Default implementation of {@link WebFilterChain}.
 *
 * <p>Each instance of this class represents one link in the chain. The public
 * constructor {@link #DefaultWebFilterChain(WebHandler, List)}
 * initializes the full chain and represents its first link.
 *
 * <p>This class is immutable and thread-safe. It can be created once and
 * re-used to handle request concurrently.
 *
 * @author Rossen Stoyanchev
 * @since 5.0
 */
public class DefaultWebFilterChain implements WebFilterChain {

    private final List<WebFilter> allFilters;

    private final WebHandler handler;

    @Nullable
    private final WebFilter currentFilter;

    @Nullable
    private final DefaultWebFilterChain next;


    /**
     * Public constructor with the list of filters and the target handler to use.
     *
     * @param handler the target handler
     * @param filters the filters ahead of the handler
     * @since 5.1
     */
    public DefaultWebFilterChain(WebHandler handler, List<WebFilter> filters) {
        Assert.notNull(handler, "WebHandler is required");
        this.allFilters = Collections.unmodifiableList(filters);
        this.handler = handler;
        DefaultWebFilterChain chain = initChain(filters, handler);
        this.currentFilter = chain.currentFilter;
        this.next = chain.next;
    }

    /**
     * Private constructor to represent one link in the chain.
     */
    private DefaultWebFilterChain(List<WebFilter> allFilters, WebHandler handler,
                                  @Nullable WebFilter currentFilter, @Nullable DefaultWebFilterChain next) {

        this.allFilters = allFilters;
        this.currentFilter = currentFilter;
        this.handler = handler;
        this.next = next;
    }

    /**
     * Public constructor with the list of filters and the target handler to use.
     *
     * @param handler the target handler
     * @param filters the filters ahead of the handler
     * @deprecated as of 5.1 this constructor is deprecated in favor of
     * {@link #DefaultWebFilterChain(WebHandler, List)}.
     */
    @Deprecated
    public DefaultWebFilterChain(WebHandler handler, WebFilter... filters) {
        this(handler, Arrays.asList(filters));
    }

    private static DefaultWebFilterChain initChain(List<WebFilter> filters, WebHandler handler) {
        DefaultWebFilterChain chain = new DefaultWebFilterChain(filters, handler, null, null);
        ListIterator<? extends WebFilter> iterator = filters.listIterator(filters.size());
        while (iterator.hasPrevious()) {
            chain = new DefaultWebFilterChain(filters, handler, iterator.previous(), chain);
        }
        return chain;
    }

    public List<WebFilter> getFilters() {
        return this.allFilters;
    }

    public WebHandler getHandler() {
        return this.handler;
    }


    @Override
    public Mono<Void> filter(ServerWebExchange exchange) {
        return Mono.defer(() ->
                this.currentFilter != null && this.next != null ?
                        this.currentFilter.filter(exchange, this.next) :
                        this.handler.handle(exchange));
    }

}
